ar X iv : a st ro - p h / 06 08 59 3 v 2 1 4 D ec 2 00 6 Dusty waves on a starry sea : the mid - infrared view of M 31

Mid-infrared observations of the Andromeda galaxy, M31, obtained with the Infrared Array Camera on board the Spitzer Space Telescope are presented. The image mosaics cover areas of approximately 3. • 7 × 1. • 6 and include the satellite galaxies M32 and NGC 205. The appearance of M31 varies dramatically in the different mid-infrared bands, from the smooth bulge and disk of the old stellar population seen at 3.6 µm to the well-known '10 kpc ring' dominating the 8 µm image. The similarity of the 3.6 µm and optical isophotes and nearly constant optical-mid-infrared color over the inner 400 ′′ confirms that there is no significant extinction at optical wavelengths in M31's bulge. The nuclear colors indicate the presence of dust but not an infrared-bright active galactic nucleus. The integrated 8 µm non-stellar luminosity implies a star formation rate of 0.4M ⊙ yr −1 , consistent with other indicators that show M31 to be a quiescent galaxy.